This invention relates to the transmission of electric power and, more particularly, to a test terminal for use in the electrical testing of a lightning arrestor without the disconnection of the transmission line conductor therefrom.
Electric power is transmitted at a variety of voltages. With respect to the transmission of power by systems operating at very high voltages, such as 345 kV (kilovolts), the physical sizes of the components of the system are massive compared to the size of the men and machinery which are employed in the installation and checkout of the equipment. Certain tests which are to be performed require the isolation of the component under test from the balance of the transmission system. However, the massive size of the components virtually precludes disconnection and reconnection of the components. For example, with reference to an electrical bushing located on the case of a transformer, circuit-breaker or other such transmission component, the electrical testing is performed by the application of a test voltage between a central conductor of the bushing and a point external to the insulation thereof. The connection of the test circuitry requires that the high-voltage electrical conductor be insulated from the test circuitry in order to insure that the test current flows through the bushing under test, and that any stray currents as may be induced by conductive pick-up from the high magnetic fields of the electric components be accounted for.
The testing situation is greatly facilitated by the use of a test terminal as is disclosed in the U.S. Pat. No. 3,363,172 issued in the name of F. C. Doble, such a test terminal often being referred to as the Doble terminal. The Doble terminal is inserted in series with the bushing, the Doble terminal being connected between the top point of the bushing and the high-voltage electrical conductor. In order to avoid the difficult and time-consuming task of disconnection of an electrical conductor for the testing of the bushing, the Doble terminal is constructed as an insulator equipped with a detachable plate-shaped conductor which bypasses the insulator to serve as a current path during normal operation of the transmission system. During testing, the bypass conductor is detached to break the connection between the bushing and the high-voltage electrical conductor. The insulator is provided with a central electrode, referred to as a guard electrode, which is utilized for the conduction of stray currents in a measurement circuit, disclosed in the foregoing Doble patent, so as to account for induced currents which would otherwise alter the measured value of the current through the bushing under tests.
A feature of the Doble terminal is found in the plate-shaped conductor which bypasses the electric current past the insulator. Since current on the order of a few thousand amperes may pass through the bypass conductor, the conductor, as well as the fittings at the ends of the Doble terminal, must be sufficiently large in cross section to accommodate the large current on a continuous basis of current flow without excessive heating. It is also desirable that the bypass conductor and the fittings be made of copper so as to reduce the resistance to the flow of electric current. As a result, the Doble terminal becomes quite heavy, and must be ruggedly constructed to withstand its own weight, as well as to provide a reliable path to the flow of the current.
While the Doble terminal can be used for bushings on the cases of transformers, circuit-breakers, and other components which are mounted within a case and have their conductors fed through apertures in the case via the bushings, the Doble terminal is less than ideal for use with a lightning arrestor which is customarily mounted in parallel electric connection with the foregoing series connection of the bushing and the Doble terminal.
The foregoing problem arises in that the Doble terminal is both massive and expensive. The expense results, in part, from the usual manufacturing process wherein the ceramic insulator is manufactured in two sections which extend outwardly from the guard electrode. Thus, there is significant cost associated with jigging of the terminal components to permit the introduction of the cement which secures the metallic fittings and the ceramic elements together. In addition, there is significant cost associated with the substantial amount of metal such as the copper, utilized in the construction of the current path.
In the case of the lightning arrestor, there is generally no flow of current except in the case of solid-state arrestors constructed of metal-oxide layers wherein there is a steady current of less than a milliampere. The major current, associated with a surge of lightning, occurs only for an instant after which the current drops back to the foregoing sub-milliampere value. It suffices, therefore, in the construction of a test terminal to modify the foregoing construction of the Doble test terminal to use substantially less metal, to use a lightweight metal, and to otherwise modify the construction of the test terminal so as to avoid the large cost and large weight of the above-described Doble terminal.